September 18, 2012

As The Temperature Rises, So Too Does Tropical Rainfall

Extreme precipitation events in the tropics can come in many forms; thunderstorm complexes, flood-inducing monsoons and wide-sweeping cyclones like the recent Hurricane Isaac.

Scientists expect global warming to intensify extreme precipitation, but the rate of intensification in the tropics remains unclear. A new study from MIT, published online this week in Nature Geoscience, gives an estimate based on model simulations and observations.

The study finds that with every 1 degree Celsius rise in temperature, tropical regions will see 10 percent heavier rainfall extremes, with possible impacts for flooding in populous regions.

“The study includes some populous countries that are vulnerable to climate change,” says Paul O´Gorman, the Victor P. Starr Career Development Assistant Professor of Atmospheric Science at MIT, “and impacts of changes in rainfall could be important there.”

Compared to other regions of the world, extreme rainfall in the tropics responds differently to climate change.

“It seems rainfall extremes in tropical regions are more sensitive to global warming,” O´Gorman says. “We have yet to understand the mechanism for this higher sensitivity.”

The impact that global warming has on rainfall in general is relatively well-understood. Carbon dioxide and other greenhouse gases entering the atmosphere increase the temperature, which in turn leads to increases in the amount of water vapor in the atmosphere. When storm systems develop, the increased humidity prompts heavier rain events that become more extreme as the climate warms.

To help understand this phenomenon, scientists around the world have been developing models and simulations of Earth's climate to show the impact of global warming on extreme rainfall around the world. For the most part, existing models do a decent job of simulating rainfall outside the tropics - for instance, in mid-latitude regions such as the United States and Europe. In those regions, the models agree on the rate at which heavy rains intensify with global warming.

When it comes to rainfall in the tropics, these models are not in agreement with one another. The reason may be resolution. Climate models simulate weather systems by dividing the globe into a grid, with each square on the grid representing a wide swath of ocean or land. Large weather systems that span multiple squares, such as those that occur in the United States and Europe in winter, are relatively easy to simulate; in contrast, however, smaller, more isolated storms that occur in the tropics may be trickier to track.

O'Gorman studied satellite observations of extreme rainfall between the latitudes of 30 degrees north and 30 degrees south — just above and below the equator. The observations spanned the extent of the satellite record, covering the last 20 years. These were compared to the observations from 18 different climate models over a similar 20-year period.

“That´s not long enough to get a trend in extreme rainfall, but there are variations from year to year,” O´Gorman says. “Some years are warmer than others, and it´s known to rain more overall in those years.”

Most of the blame for this year-to year-variability rests on El Nino – a tropical weather phenomenon that warms the surface of the Eastern Pacific Ocean. El Nino causes localized warming, changes in rainfall patterns, and occurs independent of global warming.

O'Gorman found a pattern looking through climate models that can simulate the effects of both El Nino and global warming. Models that showed a strong response in rainfall to El NiÃ±o also responded strongly to global warming, and vice versa. The results, he says, suggest a link between the response of tropical extreme rainfall to year-to-year temperature changes and longer-term climate change.

Actual rainfall satellite data over the past 20 years was compared to the model predictions for rainfall caused by El Nino and the results were consistent. The most extreme rainfall events occurred in warmer periods. Using the observations to constrain the model results, he determined that with every 1 degree Celsius rise under global warming, the most extreme tropical rainfall would become 10 percent more intense – a more sensitive response than is expected for nontropical parts of the world.

“Unfortunately, the results of the study suggest a relatively high sensitivity of tropical extreme rainfall to global warming,” O´Gorman says. “But they also provide an estimate of what that sensitivity is, which should be of practical value for planning.”

The results of the study are in line with scientists´ current understanding of how global warming affects rainfall, says Richard Allan, an associate professor of climate science at the University of Reading in England. A warming climate, he says, adds more water vapor to the atmosphere, fueling more intense storm systems.

“However, it is important to note that computer projections indicate that although the rainfall increases in the wettest regions – or similarly, the wet season – the drier parts of the tropics “¦ will become drier still,” Allan says. “So policymakers may have to plan for more damaging flooding, but also less reliable rains from year to year.”